Windows Media Player 11, Sound but no Image?

I have a couple of movies on my laptop. Whenever i try to play them on Windows Media Player 11, they do play with sound, but no image. Just like a song. I've looked up on Yahoo Answers and people have the same problem. Im thinking about Downloading Window Media Player 10 to replace 11, but im worried that the problem will continue. Any reason why WMP 11 isn't working? All answers greatly Appreciated.